Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation, i.e. a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation, thus causing a pressure gradient that forces the fluid to flow from the reservoir to the well. Often, well production is limited by poor permeability either due to naturally low permeability formations or due to formation damage that typically arises from prior well treatment, such as drilling.
To increase the net permeability of a reservoir, a well stimulation treatment can be performed. A common stimulation technique includes injecting an acid that reacts with and dissolves the damaged portion or other formation portion having naturally low permeability. The injection of acid creates alternative flow paths for the hydrocarbons to migrate through the formation to the well. The technique is known as acidizing (or more generally as matrix stimulation) and may eventually be associated with fracturing if the injection rate and pressure is sufficient to induce formation of a fracture in the reservoir.
Fluid placement is important for the success of stimulation treatments. Natural reservoirs often are heterogeneous, and the injected fluids tend to enter areas of higher permeability in lieu of entering areas where the stimulation fluid is most needed. Each additional volume of fluid follows the path of least resistance and continues to invade zones that have already been treated. Therefore, difficulty arises in placing the treating fluids in severely damaged formation zones and other low permeability formation zones.
Various techniques have been employed to control placement of treating fluids. For example, mechanical techniques involve the use of ball sealers, packers and coiled tubing placement to specifically spot the fluid across the zone of interest. Non-mechanical techniques often make use of gelling agents as diverters for temporarily impairing the areas of higher permeability and increasing the proportion of the treating fluid that flows into areas of lower permeability.